What if instead of spending an hour in the gym every other day, you could get better results from two bursts of 20 seconds at maximum effort, three times a week?

It seems an outlandish claim but that’s the idea behind the new book Fast Exercise by Dr Michael Mosley and the health writer Peta Bee. And given that Mosley is the man behind the Fast Diet, better known as the 5:2, his latest pronouncement on exercise has been commanding attention.

“This is really nothing new at all,” says Mosley, who trained as a doctor before moving into journalism and now regularly fronts science and health programmes for the BBC. “Fast exercise is just what we used to do as hunter-gatherers – doing short bursts of intense exercise, having a rest, but also incorporating it into your general life.”

He adds that High Intensity Training (HIT), which he champions in the book, has been used in its modern form for decades by athletes and fitness fanatics, but has yet to be fully assimilated into ordinary people’s exercise routines.

The advantage of HIT over extended, steady exercise is three-fold, Mosley says. “When you do HIT, you massively increase the number of mitochondria [so-called “cellular power plants” which convert fat and sugar in the blood into energy] and their efficiency. Without the intensity, that doesn’t really happen.